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|
{-# LANGUAGE OverloadedStrings #-}
-- | This module provides a representation of a @GraphQL@ Schema in addition to
-- functions for defining and manipulating Schemas.
module Data.GraphQL.Schema
( Schema
, Resolver
, Subs
, object
, object'
, objectA
, objectA'
, scalar
, scalarA
, array
, array'
, arrayA
, arrayA'
, enum
, enumA
, resolve
-- * AST Reexports
, Field
, Argument(..)
, Value(..)
) where
import Control.Monad (MonadPlus(..))
import Control.Monad.Trans.Class (lift)
import Control.Monad.Trans.Except (runExceptT)
import Data.Foldable ( find
, fold
)
import Data.GraphQL.Error
import Data.List.NonEmpty (NonEmpty)
import Data.Maybe (fromMaybe)
import qualified Data.Aeson as Aeson
import Data.HashMap.Strict (HashMap)
import qualified Data.HashMap.Strict as HashMap
import Data.Text (Text)
import qualified Data.Text as T
import Language.GraphQL.Trans
import Data.GraphQL.AST.Core
-- | A GraphQL schema.
-- @f@ is usually expected to be an instance of 'Alternative'.
type Schema m = NonEmpty (Resolver m)
-- | Resolves a 'Field' into an @Aeson.@'Aeson.Object' with error information
-- (or 'empty'). @m@ is usually expected to be an instance of 'MonadPlus'.
data Resolver m = Resolver
Text -- ^ Name
(Field -> CollectErrsT m Aeson.Object) -- ^ Resolver
type Fields = [Field]
type Arguments = [Argument]
-- | Variable substitution function.
type Subs = Name -> Maybe Value
-- | Create a new 'Resolver' with the given 'Name' from the given 'Resolver's.
object :: MonadPlus m => Name -> ActionT m [Resolver m] -> Resolver m
object name = objectA name . const
-- | Like 'object' but also taking 'Argument's.
objectA :: MonadPlus m
=> Name -> (Arguments -> ActionT m [Resolver m]) -> Resolver m
objectA name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld@(Field _ _ _ flds) resolver = withField name (resolve resolver flds) fld
-- | Create a named 'Resolver' from a list of 'Resolver's.
object' :: MonadPlus m => Name -> ActionT m [Resolver m] -> Resolver m
object' name = objectA' name . const
-- | Like 'object'' but also taking 'Argument's.
objectA' :: MonadPlus m
=> Name -> (Arguments -> ActionT m [Resolver m]) -> Resolver m
objectA' name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld@(Field _ _ _ flds) resolver = withField name (resolve resolver flds) fld
-- | A scalar represents a primitive value, like a string or an integer.
scalar :: (MonadPlus m, Aeson.ToJSON a) => Name -> ActionT m a -> Resolver m
scalar name = scalarA name . const
-- | Like 'scalar' but also taking 'Argument's.
scalarA :: (MonadPlus m, Aeson.ToJSON a)
=> Name -> (Arguments -> ActionT m a) -> Resolver m
scalarA name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld@(Field _ _ _ []) result = withField name (return result) fld
resolveRight _ _ = mzero
array :: MonadPlus m => Name -> ActionT m [[Resolver m]] -> Resolver m
array name = arrayA name . const
-- | Like 'array' but also taking 'Argument's.
arrayA :: MonadPlus m
=> Name -> (Arguments -> ActionT m [[Resolver m]]) -> Resolver m
arrayA name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld@(Field _ _ _ sels) resolver
= withField name (traverse (`resolve` sels) resolver) fld
-- | Like 'object'' but taking lists of 'Resolver's instead of a single list.
array' :: MonadPlus m => Name -> ActionT m [[Resolver m]] -> Resolver m
array' name = arrayA' name . const
-- | Like 'array'' but also taking 'Argument's.
arrayA' :: MonadPlus m
=> Name -> (Arguments -> ActionT m [[Resolver m]]) -> Resolver m
arrayA' name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld@(Field _ _ _ sels) resolver
= withField name (traverse (`resolve` sels) resolver) fld
-- | Represents one of a finite set of possible values.
-- Used in place of a 'scalar' when the possible responses are easily enumerable.
enum :: MonadPlus m => Name -> ActionT m [Text] -> Resolver m
enum name = enumA name . const
-- | Like 'enum' but also taking 'Argument's.
enumA :: MonadPlus m => Name -> (Arguments -> ActionT m [Text]) -> Resolver m
enumA name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld resolver = withField name (return resolver) fld
resolveFieldValue :: MonadPlus m
=> ([Argument] -> ActionT m a)
-> (Field -> a -> CollectErrsT m (HashMap Text Aeson.Value))
-> Field
-> CollectErrsT m (HashMap Text Aeson.Value)
resolveFieldValue f resolveRight fld@(Field alias name args _) = do
result <- lift $ runExceptT . runActionT $ f args
either resolveLeft (resolveRight fld) result
where
resolveLeft err = do
_ <- addErrMsg err
return $ HashMap.singleton (fromMaybe name alias) Aeson.Null
-- | Helper function to facilitate 'Argument' handling.
withField :: (MonadPlus m, Aeson.ToJSON a)
=> Name -> CollectErrsT m a -> Field -> CollectErrsT m (HashMap Text Aeson.Value)
withField name v (Field alias _ _ _)
= HashMap.singleton aliasOrName . Aeson.toJSON <$> runAppendErrs v
{- TODO: Report error when Non-Nullable type for field argument.
else return (HashMap.singleton aliasOrName Aeson.Null) -}
where
aliasOrName = fromMaybe name alias
-- | Takes a list of 'Resolver's and a list of 'Field's and applies each
-- 'Resolver' to each 'Field'. Resolves into a value containing the
-- resolved 'Field', or a null value and error information.
resolve :: MonadPlus m
=> [Resolver m] -> Fields -> CollectErrsT m Aeson.Value
resolve resolvers = fmap (Aeson.toJSON . fold) . traverse tryResolvers
where
tryResolvers fld = mplus (maybe mzero (tryResolver fld) $ find (compareResolvers fld) resolvers) $ errmsg fld
compareResolvers (Field _ name _ _) (Resolver name' _) = name == name'
tryResolver fld (Resolver _ resolver) = resolver fld
errmsg (Field alias name _ _) = do
addErrMsg $ T.unwords ["field", name, "not resolved."]
return $ HashMap.singleton aliasOrName Aeson.Null
where
aliasOrName = fromMaybe name alias
|
